Thin Film Transistor Technology
In large area electronic circuits such as active-matrix liquid crystal display (AM-LCD) technology, active-matrix organic light emitting displays (AM-OLED) and x-ray imagers, thin film transistors (TFTs) are used to form the switching circuits. At least one thin film transistor is required to form into a pixel of the circuits.
A schematic diagram of a pixel (100) in typical AM-LCD technology is shown in FIG. 1. A TFT (101) is deposited on a first substrate (110). During operation, the TFT (101) behaves like a variable resistor with low RON in ON state and high ROFF in OFF state. The ON and OFF states of TFT are controlled by the voltage supplied from the gate select line (102) to the gate electrode (not shown) which turns ON or OFF the active channel layer (103). The gate electrode and the gate select line (102) are connected together electrically. A layer of gate insulator (104) is deposited over the gate electrode whereas an active channel (103) is deposited to cover the gate insulator (104) and overlap the gate electrode. Metal layers are then deposited to overlap part of the active channel (103) to form source (105) and drain (106). The source (105) is connected to a data line (107) and the drain (106) is connected to an ITO pixel electrode (108) which is conductive and optically transparent. When the TFT (100) is turned ON, charges supplied from the data line (107) to represent the brightness of the pixel can be allowed to flow through the source (105) to the drain (106) to reach the ITO pixel electrode (108). These charges will be used to turn on liquid crystal or to turn on OLED materials. Majority of the ITO electrode (108) thus defines the transmissive area (109) which is smaller than the total area (111). To form the LCD units, a second substrate (not shown) with a conductive and optically transparent layer is brought over the first substrate (110), with the conductive layer on the second substrate facing the surface of the first substrate (110) where the TFT (101) and pixel electrodes (108) are deposited. The rise time of the pixel voltage when turned on should be short while the time to hold the charges (charge retention time) should be long. In order to achieve long enough charge retention time, a storage capacitor CS (112) with capacitance value substantially larger than that of the liquid crystal cell is usually inserted into each pixel to minimize the unwanted discharging. As the main components in the backplanes of AM-LCDs, the device performance of TFTs often determines the quality of the displays, such as response times and contrast ratios. Therefore, the search for high performance TFTs has never stopped.
Electrical Performance Requirements for TFTs
The parameters important to the operation of a TFT include response time, threshold voltage and ON/OFF current ratio (ION/IOFF). For TFTs utilizing active channel semiconductors having high carrier mobility, the values of ION are large enough to reduce the resistance and to ensure a short RC time constant during charging of the pixel, which is equal to RON×CS (where RON is the ON resistance of the TFT and CS is the capacitance of the storage capacitor (112)). However, for TFTs with high mobility active channels, the values of IOFF are often more sensitive to processing conditions and can be quite large even with a small amount of unwanted remnant chargers in the channels. These remnant chargers can be induced by fixed charges in the gate insulator (104) adjacent to the bottom surface region of the active channel (103) or by fixed charges in a passivation layer on the top surface region of the active channel. In addition to the values of IOFF in the TFTs, the control of threshold voltage is also difficult to achieve. In many applications, it is preferable to control the value of the threshold voltage substantially to close to 0 volt. This is required so that the TFTs can be turned on by applying a voltage (positive or negative) and can be turned off by applying a reversed voltage (negative or positive). The principles are made more clear by referring to FIG. 2a, where a cross-sectional view of an example TFT (200) on a substrate (201) is shown to have a metal gate electrode (202), an gate insulator (203), a source (204), a drain (205) and an active channel (206) of n-type semiconductor. To simplify the explanation, a bottom gate electrode TFT structure has been adopted in FIG. 2a. In an ideal situation, first negative charges (207) will be induced in the active channel (103) when positive charges (208) are induced in the bottom gate electrode (202) by a first voltage (or gate voltage) with the positive polarity applied to the metal gate electrode (202) and with the negative polarity to the source (204). When a second voltage (or drain voltage) is applied between the drain (205) and source (204), a current will flow from the drain to the source and the TFT (200) is in an ON state. When a reversed gate voltage is applied with the negative polarity to the metal gate electrode (202) and the positive polarity to the source (204), the TFT (200) is turned off. Although the TFT is in an OFF state, a minimum but unwanted amount of negative charges will be induced and a minimum current, IOFF, will flow with the second voltage applied between the drain (205) and source (204). Ideally, the value of IOFF should be as small as possible in order to achieve a long charge retention time.
In many practical TFTs, the gate insulators used are silicon nitrides or silicon oxides formed by a PECVD method. For sufficient operation of the TFT, the key requirement for the gate insulator is a high breakdown electric field. However, in gate insulators made of PECVD silicon nitrides or silicon oxides there are always positive fixed charges (209) as depicted in FIG. 2b. The density of the positive fixed charges (209) is often large especially for those oxides and nitrides with high breakdown electric fields. For instance, the density of the positive fixed charges can be as high as 1012 cm−2. These positive fixed charges (209) will induce same density of second negative charges (210) in the bottom surface region of the channel layer (206) even without the application of a gate voltage. Therefore, a current will flow from the drain (205) to the source (204) when a second voltage is applied across them and the TFT (200) is in an ON state even without the application of the gate voltage.
When a positive gate voltage is applied with the positive polarity to the gate electrode (202) and the negative polarity to the source (204), certain amount of the first negative charges (207, shown in FIG. 2a) will be induced in the active channel (206) on top of the already existing second negative charges (210), making the resistance of the active channel (206) to be even smaller. Hence, with the second voltage applied between the drain (205) and the source (204), a current larger than that before applying the first gate voltage will flow from the drain to the source.
However, due to the existence of the second negative charges (210) induced by the positive fixed charge (209) in the gate insulator (203), the TFT (200) is already in an ON state without the application of a positive gate voltage. Therefore, a gate voltage with negative polarity and sufficiently large magnitude must be applied in order to supply negative charges (211) to the metal gate electrode (202), as depicted in FIG. 2c, to compensate the second negative charges (210) induced by the positive fixed charges (209) in the gate insulators (203). This results in a minimum amount of second negative charges (210′) in the active channel (206). Hence, the threshold voltages of the TFTs with positive fixed charges (209) in the gate insulators are often negative. In such cases, when a fixed voltage is applied between the drain and the source, the drain current will vary with the variation of the gate voltage in a manner shown by Curve A in FIG. 3.
In addition to the first and second negative charges, there are also third negative charges (221) as shown in FIG. 4a induced op surface region of the active channel (206). The third negative charges (221) are often formed during the fabrication or caused by exposure to atmosphere after the fabrication. Unlike the first and the second negative charges (207 and 210), the third negative charges (221) are not the results of induction and are fixed in nature (please check if this is true). Although the application of a negative gate voltage can remove (compensate) the first negative charges (207) and the second negative charges (210) in the active channel (206), the third negative charges (221) can not be removed as effectively. In order to simplify the situation, a TFT without positive fixed charges (209) in the gate insulator (203) and therefore exclusive of second negative charges (210) in the active channel (206), is depicted in FIG. 4b to demonstrate the effects of the third negative charges (221). When a voltage with negative polarity is applied to the metal gate electrode (202) to supply negative charges (211), majority of the first negative charges (207), induced by a previously applied positive gate voltage can be removed to result in a minimum amount of the first negative charges (207′). Therefore, as shown in FIG. 4b, even when a negative gate voltage with a large value is applied in an attempt to turn off the TFT (200), a significant portion of the third negative charges (221) remained near the top surface region of the active channel (206). This will conduct a current when the second voltage is applied across the drain (205) and the source (204) and constitutes an unwanted leakage current. Hence, when a fixed drain voltage is applied, the drain current will vary with the gate voltage in a manner as shown in Curve B in FIG. 3 where the OFF current for the TFT with the third negative charges (221) is substantially larger compared to that for a TFT without the third negative charges (221) (shown by curve A). As a direct result of the third negative charges (221) in the active channel, the ION/IOFF ratio is reduced. These third negative charges (221) may be caused by the ions or polar molecules adhere to the top surface of the active channel (206) during and after fabrication.
As illustrated in FIG. 4c, a fourth negative charges (222) may also be induced by a passivation layer (223), deposited on the top surface of the active channel (206) for protection purposes. This passivation layer (223) often contains positive fixed charges (224) and will induce the fourth negative charges (222) in the top surface region of the active channel (206). The negative charges (222) will add to the total negative charges in the channel region and affect the operation of the TFT, including threshold voltage and OFF state leakage current.